1. Field of the Invention
The present invention relates to an image signal transmitting system using the difference between image frames, which can transmit lots of information in a small amount of data by transmitting the actual moving areas which are extracted by using two change detecting masks and then whose contours and textures are encoded for human visual perception.
2. Description of the Prior Art
Generally speaking, since video has a lot of correlation along the time axis, an original image can be made by using only differences between frames, that is, a current frame can be produced by adding the difference information to a previous frame. Accordingly, a current frame can be produced from a small amount of the data transmitted by transmitting the frame differences which are the information of the differences between frames rather than by transmitting a full frame. When this characteristic is used, it is also possible to transmit a vast amount of information through a transmission channel having a small capacity.
Research for extracting the motion vectors of an object, whose size is fixed, has been carried out to utilize the correlation between image frames. But the following problems have been exposed A moving object has to always have a rigid body and a constant size. If there exist picture elements (pels) moving in different directions within the object, separate motion vectors must be extracted, since it would be considered as a different object. If the profile of the object is changed by rotation, it is then recognized as an absolutely different object.
In the meantime, research is being carried out for transmitting image signals by obtaining the differences between frames with an interest in the motion of each block after dividing a frame into blocks with a constant size, as shown in FIG. 1.
FIG. 1 shows a configuration of a conventional image signal transmitting system. A motion vector is extracted through a motion estimator 2 for estimating how much each of the blocks of a frame has moved, by using an original image of the current frame inputted from outside and an original image of the previous frame stored in a frame memory 1. A frame storing and predicting part 3 predicts the current image by compensating for motion of a reproduced image of the previous frame by using said reproduced image of the previous frame stored in its internal frame memory and the motion vectors of the motion estimator 2.
The predicted error, which is the difference between the original image of the current frame inputted and the image predicted by the frame storing and predicting part 3, is obtained by an adder 4, processed for a discrete cosine transform by a discrete cosine transformer 5, quantized by a quantizer 6 and transmitted to a variable length encoder 7.
The predicted error signal outputted from the variable length encoder 7 and the motion vector outputted from the motion estimator 2 are transmitted through a multiplexer 8 to a buffer 9. The buffer controls the rate of the data going out to a transmission channel.
In the case of the occurrence of more data than the capacity of the transmission channel, the status of the buffer is transmitted to a rate controller 10, which controls the quantizer 6 to adjust quantizing steps.
To obtain the reproduced image of the previous frame to be used for predicting the current frame, the quantized results of the quantizer 6 are transmitted through an inverse quantizer 27 and inverse discrete cosine transformer 28 and added to the reproduced image of the frame stored prior to the next previous image.
A conventional image signal transmitting system has the problem that it estimates motions in a unit of blocks and therefore can not estimate correctly the motion if other different motions exist within a block. A second problem is that the predicted errors consequently become bigger at the adder 4 and the number of bits being variable length encoded is increased. A third problem is that it is not proper to use the discrete cosine transform because the human eyes cannot be expressed perfectly in a mathematical manner.